A battery employed in an electric-powered vehicle is required to generate a high voltage for driving a travel-motion motor. Normally, the battery includes a plurality of secondary batteries connected to each other in series. FIG. 6 is a diagram showing such a configuration as disclosed in JP-A-11-248,755, the contents of which are incorporated herein by reference. Voltage detection terminals T1, T3 and T5 of voltage generators V1 to V5 connected to each other in series are connected to a first multiplexer 1 comprising switches S1, S3 and S5. Voltage detection terminals T2, T4 and T6 of the voltage generators V1 to V5 are connected to a capacitor 3 via a second multiplexer 2 comprising switches S2, S4 and S6. Output terminals of the multiplexers 1 and 2 are further connected to a voltage measurement circuit 5 through the capacitor 3 and a sample switch 4, which comprises switches 4a and 4b. 
In accordance with this conventional technology, the multiplexers 1 and 2 multiplex voltages of voltage generators V1 to V5 having their own reference electric potentials to select ones of the electric potentials so that only one voltage measurement circuit 5 is capable of sequentially measuring the selected electric potentials. It is an object of this conventional technology to simplify a flying capacitor technique traditionally adopted as described in FIG. 13 of JP-A-11-248,755.
The flying capacitor technique may be simplified by a configuration in which voltage measurement circuits are respectively provided for each of the voltage generators V1 to V5 and the voltage measurement circuits are connected in parallel to the voltage generators. With such a configuration, however, the reference electric potential of the voltage-generator circuit for generating a signal varies from circuit to circuit. This problem can be conceivably solved by selecting an electric potential of the signal by using multiplexers in a configuration disclosed in JP-A-11-248,755. In this configuration, however, since either the multiplexer 1 or the multiplexer 2 is selected, the polarity of a voltage selected from the voltages of the voltage generators V1 to V5 appears inverted alternately so that an absolute-value circuit 6 for correcting the voltage polarity is separately required. In addition, the use of a capacitor for propagation of a digital signal having a value represented by binary levels in place of an analog is not efficient.
In general, the problems can be conceivably solved by once insulating a propagated signal through the use of a device such as a photo coupler or a MOS relay prior to conversion of the voltage of the signal into a reference electric potential on the control side. However, this solution has a problem that an insulator for insulating a propagated signal is expensive and a photo-electrical conversion process carried out by the photo coupler introduces a delay in the propagation of the signal.